Many images, whether captured digitally or on traditional film, are subject to color discrepancies or variances from what the consumer expects to see, or from legally required color schemes. This can be due to, for example, incorrect photographic settings on the part of the consumer resulting in, for example, under- or over-exposing an image, improper lighting, color scene variances due to film type, development processing, print stock coloration, or use of poor-performing cameras. Further, what the human eye sees can be different from the range of colors recorded digitally or on film. In particular, the human visual system recognizes colors in relation to adjacent colors and in accordance with lighting conditions, whereas digital and film photography attempt to record the actual color of an object.
Images that have areas of white or black may appear different on a monitoring screen or in print than what the consumer expects. For example, photographic identification (ID) images for use in official documents such as driver's licenses, passports, and other identification documents, are intended to have a white background, but often appear to have a slightly colored background, whether a neutral color (for example, white, black, or gray) or colored (for example, blue-tinted or red-tinted). Snow or ice scenes can appear overall blue. It is noted that snow and ice in nature frequently do have a blue or green tint, however consumers expect to see a more white coloration of snow and ice. Darkly shaded scenes, such as forest or cave images, also are frequently distorted toward a certain color, for example, green due to foliage, and red or yellow due to lighting conditions, respectively. Other colors can also be distorted from consumer expectations.
Color balancing, altering both the color and brightness of an image, is often used to attempt to correct unwanted colorations, but can result in other coloration variances that are unacceptable. Traditional automatic scene balance algorithms tend to adjust the exposure so that the average color of the image is a neutral gray, which is acceptable for many consumer photographic images, but not for particular scenes or for meeting legal coloration requirements. For example, simply balancing a photographic ID image to make the background white frequently distorts the coloration of the subject's face, hair, and eyes, and may brighten the face so the features are washed out. Alternately, balancing towards a neutral gray color in such a scene can be affected by the coloration of the subject's clothing, for example, and produce a tinted background. Similarly, removing some of the blue from a sky, snow, or ice scene may improperly remove blue tint from other objects, such as people, cars, buildings, and foliage, altering the coloration beyond an acceptable level. This is also true for images with dark shadows. To lighten or alter the color of the shadows may affect the entire image by making it too bright, or by shifting the color of other areas of the image unacceptably. Similar unacceptable results may occur when color balance is done to correct skewing of other colors.
Color balance and other processing techniques can be done automatically by a camera, video recorder, or other imaging apparatus during or after recording an image, or during initial processing of the image, whether by physical, optical, or digital processing. Such processing may also negatively affect the resulting image, for example, by skewing the color balance based on a color that occupies much of the image, or increasing saturation levels of colors such that desirably neutral-colored areas are affected.
Color balance and achievement of a desired color in an image that is representative of the real color of the subject, or of an expected color of the subject is important in many different industries. Photographic ID images, consumer and professional photographs, and advertising media are just some examples of industries wherein color is extremely important. For example, the success of an advertisement for a clothing line, food item, corporate logo, or sports team relies on achieving an expected realistic coloration of the subject. Regardless of the final format of the image, whether achieved by display on a monitor, in print media, on a photographic medium, or some other media, it is desirable to control the visual characteristics of the image.
Methods of adjusting the backgrounds of images or scenes are known for use in document handling and photographic industries. In document handling, artifacts from copying or handling can be removed by forcing the background toward white, as described, for example, in U.S. Pat. Nos. 6,323,957; 6,222,642; and 5,157,740. In the photographic industry, particularly for photographic identification images, separation of the background from the subject, and replacement of the background with a desired image or color, is known, as described, for example, in U.S. Pat. Nos. 6,999,113; 6,950,130; 6,912,313; and 5,913,019, and WO 94/26057. Methods of transforming color-image signals while maintaining shadow and highlight characteristics are addressed in U.S. Pat. No. 6,278,800. U.S. Pat. No. 6,608,926 describes a color fog correction wherein the area of interest is removed before correction. U.S. Patent Application Publication US 2004/0071343 describes methods of correcting color fog in images using LCC color space conversions. U.S. Pat. No. 4,928,167 similarly describes tonal corrections for video signals using conversion to LCC color space.
It is desirable to have a means of correcting for a specific color within an image when a desired value of the specific color is known.